Studies are being conducted on the processes of shedding of outer segment membranes and their subsequent phagocytosis by the pigment epithelium with emphasis on control mechanisms of these processes. Phagosome counts by light microscopy and autoradiography are the major analytical techniques, although electron microscopy is used when appropriate. The shedding rhythm of albino rats persisted after five days in constant darkness but was abolished by exposure to constant light. Pinealectomy, superior cervical ganglionectomy, and hypophysectomy had no effect on the rhythm, although injections of melatonin and isoproteranol could stimulate shedding. Serotonin N-acetyltransferase activity was detected in the retina, although no diurnal rhythm was evident. Five days after animals were placed on a shifted lighting schedule, a new pattern of shedding was observed. If one eye was occluded and the rat was placed in altered lighting, the closed eye maintained the original rhythm, while the open eye responded to the new lighting conditions. RCS rats were found to have a small peak of phagocytosis one hour after the onset of light.